The invention relates to a process for the preparation of N-phosphonomethylglycine by reacting a hexahydrotriazine compound with a triacyl phosphite, and to intermediates for use in this process.
N-Phosphonomethylglycine (glyphosate), of the following formula, 
is a widely employed nonselective herbicide. A large number of processes for the preparation of phosphonomethylglycine are known. One possibility of preparing it exists in reacting hexahydrotriazine deirvatives with phosphorous esters. Thus, U.S. Pat. No. 4,181,300 describes the preparation of hexahydrotriazines of the formula: 
and U.S. Pat. No. 4,053,505 the reaction of these hexahydrotriazines with phosphorous diesters and subsequent hydrolysis of the resulting product to give phosphonomethylglycine. It has emerged that both yield and selectivity favoring the monophosphonated product are capable of improvement. Also, phosphorous diesters are very expensive.
EP-A-104 775 (corresponding to U.S. Pat. Nos. 4,425,284, 4,482,504 and 4,535,181) describes the reaction of the above hexahydrotriazines with an acyl halide and the subsequent phosphonation with a phosphorous triester and hydrolysis to give phosphonomethylglycine in accordance with the following equation: 
While this process gives relatively good yields of phosphonomethylglycine, it requires not only the use of the expensive phosphorous esters, but additionally the use of carbonyl chloride. In addition, the carbonyl chloride might at most be recovered in the form of the free acid and then reconverted into the acid chloride in a separate step, which considerably increases the costs of the process. Moreover, the alcohol with which the phosphorous acid is esterified cannot be recycled fully since an equivalent of the corresponding alkyl chloride, which is, moreover, toxicologically unacceptable, is formed during the reaction.
U.S. Pat. No. 4,428,888 (corresponding to EP-A-149 294) describes the reaction of the abovementioned hexahydrotriazine with a phosphorous acid chloride in the presence of a strong anhydrous acid, for example hydrogen chloride, and a C1-C6-carboxylic acid, such as acetic acid. In this manner, a large number of undefined by-products are obtained, and these reduce the phosphonomethylglycine yield and necessitate complicated purification of the product.
U.S. Pat. No. 4,442,044 describes the reaction of a hexahydrotriazine of the formula 5 with a phosphorous acid triester to give the corresponding phosphonate compound, which is used as herbicide.
DD-A-141 929 and DD-A-118 435 describe the reaction of an alkali metal salt of the above hexahydrotriazine (R=for example Na) with a phosphorous diester. However, since alkali salts are sparingly soluble, the conversion rate is only low.
U.S. Pat. No. 5,053,529 describes the preparation of phosphonomethylglycine by reacting the above hexahydrotriazines with phosphorous triesters in the presence of titanium tetrachloride, followed by hydrolysis of the product obtained. The use of titanium tetrachloride makes the preparation considerably more expensive. Moreover, the phosphonomethylglycine yields are unsatisfactory.
U.S. Pat. Nos. 4,454,063, 4,487,724 and 4,429,124 describe the preparation of phosphonomethylglycine by reacting a compound of the formula 
in which R1 and R2 are aromatic or aliphatic groups with RCOX (Xxe2x95x90Cl, Br, I) to give a compound of the formula 
reaction of this product with a metal cyanide and hydrolysis of the product obtained. The disadvantages of this method are stated as above in relation to the use of the acid chloride.
Other possible syntheses which have been described are based on the cyanomethyl-substituted hexahydrotriazine of the formula. 
U.S. Pat. Nos. 3,923,877 and 4,008,296 disclose the reaction of this hexahydrotriazine derivative with a dialkyl phosphonate in the presence of an acidic catalyst such as hydrogen chloride, a Lewis acid, a carbonyl chloride or a carboxylic anhydride to give a compound of the formula: 
Subsequent hydrolysis yields the phosphonomethylglycine, 8 to 10% of the diphosphonomethylated product being formed.
U.S. Pat. Nos. 4,067,719, 4,083,898, 4,089,671 and DE-A-2751631 describe the reaction of cyanomethyl-substituted hexahydrotriazine with a diaryl phosphonate in the absence of a catalyst to give a compound 9 where Rxe2x80x3=aryl. This method has he same disadvantages as described above for the use of the carboxyl-substituted hexahydrotriazine 5.
EP-A-097 522 (corresponding to U.S. Pat. Nos. 4,476,063 and 4,534,902) describes the reaction of the hexahydrotriazine 6 with an acyl halide to give 10, the subsequent phosphonation with a phosphorous triester or diester to give 11 and finally the hydrolysis to phosphonomethylglycine as described in the following equation: 
The disadvantages are the same as for the processes in which the carboxyl-substituted hexahydrotriazine derivatives are used.
Finally, U.S. Pat. No. 4,415,503 describes the reaction of the cyanomethyl-substituted hexahydrotriazine in a manner which is similar to the process described in U.S. Pat. No. 4,428,888. Again, substantial formation of by-products can be observed.
EP 164 923 A describes an improved hydrolysis of a compound of the formula 11.
It is an object of the present invention to provide a simple and economical process for the preparation of phosphonomethylglycine in which the phosphomethylglycine is additionally obtained in high purity.
We have found that this object is achieved by reacting a hexahydrotriazine derivative with a triacyl phosphite and subsequently hydrolyzing the product obtained to give phosphonomethylglycine.
The present invention therefore relates to a process for the preparation of N-phosphonomethylglycine, wherein
a) a hexahydrotriazine derivative of the formula II 
xe2x80x83in which X is CN, COOZ, CONR1R2 or CH2OY,
Y is H or a radical which is readily exchangable for H,
Z is H, an alkali metal, alkaline earth metal, C1-C18-alkyl or aryl, which is unsubstituted or substituted by C1-C4-alkyl, NO2 or OC1-C4-alkyl,
R1 and R2 can be identical or different and are H or C1-C4-alkyl, is reacted with a triacyl phosphite of the formula III
P(OCOR3)3
in which the radicals R3, which can be identical or different, are C1-C18-alkyl or aryl which is unsubstituted or substituted by C1-C4-alkyl, NO2 or OC1-C4-alkyl,
to give a compound of the formula I 
in which R3 and X have the abovementioned meanings and
b) the compound of the formula I is hydrolyzed, and, if X is CH2OY, oxidized.
Furthermore, the invention relates to the compounds of the formula I and their preparation in accordance with step a) of the process for the preparation of phosphonomethylglycine.
Alkyl is a linear or branched alkyl chain having preferably 1 to 8 carbon atoms and in particular 1 to 4 carbon atoms. Examples of alkyl are methyl, ethyl, n-propyl, i-propyl, n-butyl, 1-butyl, sec-butyl, t-butyl, n-hexyl, 2-ethylhexyl, etc.
Aryl is preferably phenyl and naphthyl.
X is preferably CN or COOZ.
Z is preferably H, alkali metal or C1-C18-alkyl.
If Y represents a radical which is readily exchangable for H, this preferably takes the form of an aliphatic or aromatic acyl radical or C1-C6-alkyl group. The aliphatic acyl radical is preferably a C1-C6xe2x80x94CO radical, and the aromatic acyl radical preferably takes the form of the benzoyl radical.
R1 and R2 are preferably H.
The radical R3 especially preferably takes the form of an aryl radical which can be unsubstituted or substituted as stated above. Especially suitable radicals R3 are phenyl, p-tolyl and p-nitrophenyl.
The compounds of the formula II are known and can be prepared in the known manner or analogously to known processes, see, for example, the state of the art mentioned at the outset. For example, an amine X-CH2xe2x80x94NH2 can be reacted with a formaldehyde source, such as aqueous formalin solution or paraformaldehyde, for example by dissolving the primary amine in the aqueous formalin solution. The desired hexahydrotriazine can subsequently be obtained by crystallization or evaporation of the water. This process is described in DE-A-2645085 in accordance with U.S. Pat. No. 4,181,800, whose full extent is referred to herewith.
The compound of the formula II in which X is CN can be obtained by Strecker synthesis, i.e. by reacting ammonia, hydrocyanic acid and a formaldehyde source. Such a process is described, for example, in U.S. Pat. No. 2,823,222, whose full extent is referred to herewith.
The compounds of the formula III can be prepared by a plurality of processes. A first possibility is to react a salt of a carboxylic acid R3COOH with a phosphorus trihalide, in particular phosphorus trichloride. The carboxylate used is preferably an alkali metal salt or alkaline earth metal salt, in particular the sodium, potassium or calcium salt, or the ammonium salt. This reaction can be carried out without using a solvent, and the reaction product obtained can be used directly in step (a). However, the process is preferably carried out in an inert organic solvent, in particular in an ether such as dioxane, tetrahydrofuran and the like, a halogenated, in particular a chlorinated or fluorinated, organic solvent such as dichloromethane, 1,2-dichloroethane, 1,2-dichloropropane, 1,1,1-trichloroethane, 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane, chlorobenzene or 1,2-dichlorobenzene, an aliphatic or aromatic hydrocarbon such as n-octane, toluene, xylene or nitrobenzene. It is preferred to use the same solvent as subsequently in step (a). Especially preferred is the use of a chlorinated hydrocarbon.
The salt formed during the reaction, for example sodium chloride when using phosphorus trichloride and the sodium salt of the carboxylic acid employed, can be eliminated after the reaction. If ammonium chloride or another ammonium halide is obtained as the salt, the ammonia employed can be recovered by basifying (pH 11-14) an aqueous solution of the salt with a strong base, for example sodium hydroxide solution, and subsequently stripping the ammonia in the customary manner. The ammonia obtained in this manner can be recirculated after drying, for example by distillation in the liquid or gaseous state, or as aqueous solution, and used for preparing the ammonium salt of the carboxylic acid.
Another possibility of preparing the compounds of the formula III is to react a carboxylic acid R3COOH with the phosphorus trihalide in the presence of an amine. Amines which are used are, in particular, aliphatic or cycloaliphatic di- or triamines, such as triethylamine, tributylamine, dimethylethylamine or dimethylcyclohexylamine, and pyridine. In general, such a procedure is carried out in an organic solvent. Suitable solvents are stated above in connection with the first possibility for preparation. Dioxane, 1,2-dichloropropane, 1,2-dichloroethane, nitrobenzene or toluene are preferably used. When using a solvent, the amine hydrochloride formed precipitates and can be removed by filtration. If the amine hydrochlorides are treated with a strong base, for example with aqueous sodium hydroxide solution, the amines are set free from the hydrochloride. Volatile amines can then be recovered by distillation or extraction. Nonvolatile amines can be recovered by extraction or, if a two-phase mixture is obtained when the amine is set free, by phase separation. Solid amines can be recovered by filtration. The amines recovered can be recirculated into the process, if appropriate after drying.
Another possibility of preparing the compounds of the formula III is to react the carboxylic acid R3COOH with a phosphorus trihalide, in particular phosphorus trichloride, without addition of a base. In this reaction, the hydrogen halide which forms must be removed from the reaction mixture. This can be done in the customary fashion, for example by passing through an inert gas, such as nitrogen. The hydrogen halide which has been set free can then be used for the hydrolysis in step (b) in the form of an aqueous solution.
Step (a) of the process according to the invention can be carried out with or without solvent, for example in the melt. However, it is preferred to use an inert organic solvent, for example a hydrocarbon such as toluene or xylene, an ether such as tetrahydrofuran, dioxane or dibutyl ether, nitrobenzene and the like. It is especially preferred to carry out the process in a halogenated solvent, in particular a chlorinated, preferably a chlorinated and/or fluorinated aliphatic hydrocarbon, such as dichloromethane, 1,2-dichloroethane, 1,2-dichloropropane, 1,1,1-trichloroethane, 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane, chlorobenzene or 1,2-dichlorobenzene. The reactants are expediently employed in essentially stoichiometric amounts. However, an excess of, for example, up to 10% of one or the other reactant may also be used. In general, the reaction temperature is in the range of xe2x88x9210xc2x0 C. to 140xc2x0 C., preferably in the range of room temperature to 100xc2x0 C. Only short reaction times are required under these conditions; in general, the reaction is essentially complete after 10 to 30 minutes.
The compounds of the formula I obtained in accordance with step (a) are useful intermediates for preparing phosphonomethyl-glycine. To this end, the compounds of the formula I are hydrolyzed. The hydrolysis can take the form of an acid or alkaline hydrolysis, preferably an acid hydrolysis. Acids which are used are, in particular, inorganic acids such as hydrochloric acid, sulfuric acid or phosphoric acid. Alkaline hydrolysis is generally carried out using an alkali metal hydroxide or alkaline earth metal hydroxide, in particular using sodium hydroxide or potassium hydroxide.
The hydrolysis is expediently carried out with an aqueous acid or base. In general, the aqueous acid or base is added to the reaction mixture obtained in step (a). The hydrolysis can be carried out without solvent or in the presence of an inert organic solvent which is fully or in part or not miscible with water. The solvent employed in step (a) is preferably used. When using a solvent in step (a), the reaction mixture obtained in step (a) is expediently employed directly, if appropriate after removing, for example by distillation, some of the solvent. As an alternative, the solvent used in step (a) is removed completely and the residue is hydrolyzed. The solvent recovered from the reaction mixture can be reused in the preparation of the compounds of the formula III or in step (a).
It is especially preferred to carry out the hydrolysis in a two-phase system (aqueous phase/organic phase). Here, an organic solvent which is miscible in part with water or not miscible with water is used, preferably a hydrocarbon such as toluene or xylene, an ether such as dibutyl ether and in particular a halogenated hydrocarbon such as stated above as solvent for step (a). Hydrolysis is carried out with intimate mixing of the two phases using customary devices, for example stirred reactors, circulating reactors or, preferably, static mixers. After hydrolysis has ended, the phases are separated and processed as described below.
An especially preferred embodiment is a process in which step (a) is carried out in a halogenated solvent, some of the solvent is removed, if appropriate, and the resulting compound of the formula I is hydrolyzed by treating the reaction mixture obtained in step (a) with an aqueous acid or base.
As an alternative, the compound of the formula I can also be hydrolyzed into phosphonomethylglycine by enzymatic means, for example using an esterase or a nitrilase.
The acid or base is used in at least equivalent amounts, but preferably in excess, in particular in an amount of xe2x89xa72 equivalents.
The temperature at which the hydrolysis is carried out is generally in the range of approximately 10xc2x0 C. to 180xc2x0 C., preferably 20 to 150xc2x0 C.
If X is CH2OY, the product obtained after hydrolysis still requires oxidation. The starting compound is, in particular, one in which X is CH2OH. The oxidation to give phosphonomethylglycine is carried out in the customary manner which is known to the skilled worker, for example by catalytic dehydration with copper catalysis.
If X is CH2OY and Y is an acyl radical, hydrolysis of the product of step a) involves elimination of the acyl radical with formation of the corresponding compound where X=CH2OH. This is oxidized as stated above to give phosphonomethylglycine.
If X is CH2OY and Y is an alkyl radical, ether cleavage normally takes place simultaneously under the conditions of an acid hydrolysis of the product of step a). The compound obtained where X=CH2OH is oxidized as stated above to give phosphonomethylglycine.
The phosphonomethylglycine obtained when hydrolyzing using an excess of acid or base is dissolved in the aqueous phase. The carboxylic acid R3COOH is formed directly when hydrolyzing with an excess of acid or, when hydrolyzing with a base, after acidification with a strong acid, preferably to a pH of  less than 0.5. The carboxylic acid is then removed in the customary manner, for example by filtration of the carboxylic acid which has precipitated in solid form, distillation or extraction with an organic solvent which is not miscible with the aqueous phase. If hydrolysis is biphasic, the carboxylic acid is present in dissolved form in the organic phase, if appropriate. The carboxylic acid is then removed by separating off the organic phase and, if desired, can be recovered therefrom in the customary manner. It is obtained in high purity and can be recycled without problems for the preparation of the compound of the formula III. The solvent which forms the organic phase can be recirculated and reused in the preparation of the compounds of the formula III or in step (a). In general, however, the solvent is previously subjected to distillation, extraction, filtration and/or stripping in order to remove contaminants such as alcohols, phenols, ammonium salts and/or carboxylic acids which are soluble or insoluble in water.
The phosphonomethylglycine can be precipitated by bringing the aqueous phase to a pH in the region of 0.5 to 2.0, in particular 0.8 to 1.5, for example by adding an acid or base, for example HCl, H2SO4 or NaOH, KOH, Ca(OH)2 and obtained in the customary manner, if appropriate by concentrating the aqueous phase and/or addition of a precipitant, for example by filtration. A solvent which is miscible with water, such as methanol, ethanol, isopropanol, acetone and the like, is preferably used as precipitant. The solvents can be recovered from the mother liquor by distillation and reused.
Ammonia or ammonium chloride formed during the hydrolysis can be recirculated into the process, if appropriate by basifying the mixture and recovering the ammonia by stripping.
If required, the phosphonomethylglycine obtained can be decolorized in the customary manner. This can be done for example by treating it with small amounts of a decolorizing agent, for example oxidants such as perborates or H2O2 or adsorbents such as active charcoal. The amount of decolorizing agent depends on the degree of the discoloration and can be determined in a simple manner by the skilled worker. The treatment with the decolorizing agent can be performed at any point after hydrolysis and in the customary manner. It is expedient to add the decolorizing agent before precipitating the phosphonomethylglycine.
The process according to the invention, or each step on its own, can be carried out continuously, discontinuously or as a semi-batch process. Reaction vessels which are customary for such purposes are used, such as stirred reactors or tubular reactors, if appropriate with mixing devices arranged upstream or with mixing elements integrated into the tubular reactor.
The process according to the invention is thus distinguished by simple process control and inexpensive feed stocks. Only an inorganic chloride is produced as waste product, and the protective groups, viz. the actyl radicals of the triacyl phosphite of the formula III, can be recycled in a simple manner. The process yields phosphonomethylglycine in very short reaction times and high yields of  greater than 90% of hexahydrotriazine of the formula II.
The examples which follow illustrate the invention without imposing any limitation.